1. Field Of The Invention
The invention relates to a continuous process for preparing acrylonitrile polymers or copolymers particularly suitable for processing into fibers.
2. Description Of The Prior Art
It is known that polyacrylonitrile or acrylonitrile copolymers containing the former in a proportion of 85% by weight at least, are widely used in the art, more particularly for producing fibers.
These polymers are prepared in the art by polymerization of acrylonitrile or by copolymerization of acrylonitrile with one or more ethylenically unsaturated compounds copolymerizable therewith in the presence of suitable catalysts, the process being normally carried out in an aqueous medium.
More particularly, known polymerization catalysts are peroxide compounds generally, such as benzoyl peroxide and alkali metal or ammonium persulphates, or combinations of these compounds with a substance of a reducing character, the combinations being known as redox systems. In the continuous operation of polymerization processes the constituents of the catalytic system, monomers and aqueous reaction medium are continuously fed to a reactor in which the reaction mass is stirred and kept at a controlled temperature. A polymer slurry is continuously discharged from the polymerization reactor, the unreacted monomers and, subsequently, the polymer being separated from the said slurry. The polymer is subjected to washing and drying treatments.
Since the periods of residence under polymerization conditions are purely statistical, operating with a plurality of reactors in series is generally preferred.
Normally, polymerization is carried out in two reactors in series, a conversion as high as possible of the monomer being effected in the former one, final conversion being then adjusted to the desired extent in the latter one acting as a finishing reactor.
As is known, the properties of the fibers obtained from polymers or copolymers of acrylonitrile depend upon the properties conferred to the polymers, such as molecular weight, distribution of the molecular weights, structure, etc.
Attempts have been made in the art at influencing the properties of the polymers either by selecting the catalytic system, or, above all, by adjusting the reaction parameters.
In the processes carried out in two polymerization steps in series, different values of the reaction parameters are normally maintained during the two steps, such as pH and temperature values, in order to influence the properties of the acrylonitrile polymers.
Moreover, as stated above, the prior art teaches adjusting conditions so as to effect at the first step conversion of the monomer as high as possible, the conversion being maintained within relatively moderate limits at the second polymerization step.
This procedure is not free from drawbacks, in that a relatively high number of parameters at a plurality of polymerization steps are not readily influenced.
Moreover, with these processes, acrylonitrile polymers of fully satisfactory properties cannot be obtained.
In commercial plants for preparing very high quantities of acrylonitrile polymers, a plurality of production lines are normally employed, because for practical reasons the capacity of the containers suitable for the polymerization reaction is limited.
However, both for economic reasons and for the sake of a more constant character of the properties of the polymers, one line of production only is desirably operated.
Finally, in an industrial process, adjustment of the quantity of the polymer produced is desirable or, in other words, the productive capacity of the polymerization plant should be varied according to particular requirements.
In known processes an adjustment of this kind is not easily obtained or, at least, the polymer production cannot be varied without causing at the same time unbalances and undesirable variations in the polymerization plant.